This invention relates to a method of impregnating spheres of activated alumina with cerium, the spheres being useful in catalyst supports and particularly in supports for so-called three-way catalyst for the purification of exhaust gases of internal combustion engines.
As a practically advantageous means for purifying exhaust gases discharged from automotive internal combustion engines, a great interest has been attached to so-called three-way catalysts which can accomplish both the oxidation of carbon monoxide (CO) and hydrocarbons (HC) and the reduction of nitrogen oxides (NOx) simultaneously. Usually a three-way catalyst contains a plurality of metals of the platinum group, such as platinum and rhodium, as catalytic elements which are supported on a carrier or support in the form of either spheres or a monolithic body of honeycomb structure.
In practice, a three-way catalyst is desired to have a window width as wide as possible. The term "window width" refers to a range of the composition of the exhaust gas, in which the catalyst exhibits a satisfactorily high conversion value (e.g. at least 80%) for every one of CO, HC and NOx. In this regard, it is known that the presence of cerium oxide or ceria which has an oxygen-storage function in a three-way catalyst is effective for expansion of the window width. In producing a three-way catalyst based on this knowledge, it is usual to prepare a catalyst support by utilizing small spheres of activated alumina impregnated with cerium in an oxidized state. Such alumina spheres are prepared through the steps of wetting spheres of activated alumina with an aqueous solution of a cerium salt, drying the wet spheres in a hot-air oven, and calcining the dried spheres in air. For example, a monolithic catalyst support is coated with cerium-containing activated alumina by first pulverizing the cerium-impregnated spheres of activated alumina in a suitable liquid to obtain a coating liquid, then wetting a conventional monolithic support with this coating liquid, followed by drying, and calcining the dried monolithic support. A three-way catalyst is produced by applying catalytic elements onto the surfaces of the thus coated monolithic support by a known method.
Certainly, three-way catalysts obtained by utilizing cerium-impregnated spheres of activated alumina are featuring relatively large values of the window width, but still there is a demand for further expansion of the window width of three-way catalysts.